btrfs: don't wait for writeback on clean pages in extent_write_cache_pages

btrfs: don't wait for writeback on clean pages in extent_write_cache_pages

__extent_writepage could have started on more pages than the one it was
called for.  This happens regularly for zoned file systems, and in theory
could happen for compressed I/O if the worker thread was executed very
quickly. For such pages extent_write_cache_pages waits for writeback
to complete before moving on to the next page, which is highly inefficient
as it blocks the flusher thread.

Port over the PageDirty check that was added to write_cache_pages in
commit 515f4a037fb ("mm: write_cache_pages optimise page cleaning") to
fix this.

CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>